Incident metering is a method of determining exposure that reads the light falling on a scene. This is the function of the lens covered by a white dome on handheld meters, as shown here. When you hold the meter near the subject, so that the dome is directed toward the camera position, it reads the ambient light and gives you incredibly precise exposure information regardless of the reflectivity of the subject. This is extremely important, because the reading isn’t influenced—or tainted—by the subject in any way. This means that accuracy isn’t compromised by what the subject looks like. If you are shooting snow, as in the forest scene below, or a subject in front of a black background with studio lighting like model of a T. rex skeleton, the meter only reads the light emanating from the light source.

The extremes in tonality, from the brilliant white snow to the blackness of the velvet, aren’t part of the equation when the meter calculates the light intensity. Whether you are taking a reading outdoors in bright sunlight, indoors with a household lamp, or in a warehouse lit with fluorescent or mercury vapor lights, the incident function on the meter performs flawlessly.

The metering method built into your camera, reflected metering, reads the light reflected from a scene. You can set the camera so the meter gives more weight to certain areas of the viewfinder when evaluating light by selecting modes such as spot (to read the center 3% to 5%), center-weighted (the meter reads the entire scene but gives more weight to the center), partial (the meter reads the center 9%), or matrix (the frame is divided up into sections and each one is assigned a value). All of them read the light that is being reflected from the subject or scene.

This works very well if your subject is in the center of the viewfinder and is essentially middle toned, as in this picture of a 1936 Auburn.

The classic car and the grass around it are middle toned (or middle gray, if the image is rendered in black-and-white), and you can expect an accurate reading from this composition. It is true that the sky is much brighter, but since the white sky was not placed near the center of the composition, it will have very little influence on the meter reading. Similarly, the picture below of my dog Rexie’s first birthday party is mostly middle toned except for Rexie and the driveway. Notice, though, that he was positioned off-center and the driveway is at the right edge of the frame. Since built-in meters gather most of the light values in a picture from the center, Rexie isn’t influencing the meter very much, and therefore I could expect a reasonably good exposure. However, when I photographed Rexie in the snow, the entire center of the frame was filled with his pure white fur, and the built-in meter predictably underexposed the image.

By contrast, an incident meter produced a perfect exposure in both situations.

Cameras don’t have a built-in incident metering system; they only use reflected metering. This is why many photographers are so insecure about their exposures. Even if they know the exposure will not be correct, it’s difficult to determine how many f-stops or fractions of an f-stop the meter will be off. Using the incident function on a handheld meter takes away the insecurity of most lighting scenarios.

The only type of light an incident meter can’t read is backlighting. To calculate a correct exposure for that kind of situation, a reflected meter reading is required. The picture below of Kicker Rock in the Galapagos Islands was backlit at sunrise, and in order to read the light in this situation I used a handheld meter with its 1-degree spot mode capability. I switched from incident mode to reflected mode and determined the exposure by reading the darker portion of the blue sky at the top of the frame. In this situation, the built-in light meter in a camera used in spot mode would have given me the same results.

On the other hand, for the silhouette of the Masai in Kenya, below, I would only use the reflected mode in a handheld meter because it is much more precise. It allowed me to pinpoint the tone in the sky that was closest to middle gray. I couldn’t risk using the built-in meter in the camera because if it had been wrong, I might not have had time to make corrections before this unique moment was lost.